Process for making quartz glass casings



April 7, 1964 W. H. HERAEUS ETAL PROCESS FOR MAKING QUARTZ GLASS CASINGSOriginal Filed Nov. 25, 1953 INVENTORS W/LHELM HEINRICH HERAEUS HEINRICHMOHN United States Patent PROCESS FOR MAKING QUARTZ GLASS CASINGSWilhelm Heinrich Heraeus, Hanan am Main, and Heinrich Mohn, Hailer,Kreis Gelnhausen, Germany, assignors to Heraeus Quarzschmelze G.m.b.H.,Hanau am Main, Germany, a corporation of Germany Original applicationNov. 25, 1953, Ser. No. 394,462, now Patent No. 2,904,713, dated Sept.15, 1959. Divided and this application Oct. 15, 1958, Ser. No. 767,364Claims priority, application Germany Nov. 27, 1952 4 Claims. (Cl.65-111) This invention relates to quartz lamps used both for therapeuticand visible lighting or illumination purposes. This application is adivision of application Serial No. 394,462, filed Nov. 25, 1953, nowPatent 2,904,713, as a continuation-in-part of corresponding applicationSerial No. 367,687, filed July 13, 1953, now Patent 2,954,496.

One object of this invention is to produce quartz lamps of high emissionof energy or light, both ultra-violet and illuminating, and maintainingsuch emissions for a long time at high efiiciency.

Another object of this invention is to provide quartz glass casings forgas discharge tubes, said casings being made from substantially purequartz glass and imparting to gas discharge tubes and lamps a prolongedservice life and other valuable properties.

A further object of this invention is to provide a process of makingsuch casings from quartz glass.

Still another object of this invention is to provide a process ofproducing substantially pure quartz glass which is suitable not only formaking casings of gas discharge tubes and lamps but also for makingother quartz glass articles and quartz glass parts of articles andapparatus useful, for instance, for optical instruments and the like.

A further object of this invention is to provide casings of quartz glassfor gas discharge tubes which casings are made from quartz glasssubjected to a process whereby foreign elements having a harmful effectupon the service life of such gas discharge tubes are rendered harmless.

Other objects of this invention and advantageous features thereof willbecome apparent as the description proceeds.

Quartz lamps of the high pressure mercury vapor discharge type have beenwidely used for therapeutic purposes and lately have come into extensiveuse in street lighting systems. Such quartz lamps, in all manner of use,lose a substantial part of their efi'iciency by reason of thesubstantial deterioration in the light transmission efficiency of thequartz glass casing or envelop.

Thus ultra-violet therapeutic lamps, made of quartz glass, rated at 100%efiiciency when first used, may for instance lose substantially 40% oftheir ultra-violet ray emission efiiciency after about 1500 hours of usealthough the ultra-violet ray producing source of the lamp still hasmany hundreds of additional hours of useful operation. A similarsubstantial loss occurs in the visible light emission with quartz casinglamps used for illumination purposes although there again the lightproducing source of the lamp has many hundreds of additional hours ofuseful operation. Such losses of efliciency commence almost immediatelywith the operation of the lamps and may be evident on examination afteralready 100 hours of use; after about 1500 hours of use such sufficiencymay have dropped for instance to about 40% or more from its' initialefliciency.

Efiiciency loss in street illumination is highly uneconomical as suchlamps generally have a service span of the light source of well over3000 hours. In ultra-violet therapeutic lamp uses, the deterioration isnot only costly but leaves the user without information how long thelamp can be used with relative exactness of dosage as a therapeuticalappliance.

3,128,169; Patented Apr. 7, 1964 It has been found that high resistanceto aging and a considerably prolonged service life are achieved whenusing a substantially pure quartz glass, The degree of purity of thequartz glass used for making casings for quartz lamps and the like is ofthe utmost importance with respect to a prolonged service life of suchlamps. Casings of gas discharge tubes as they are used in high pressurelamps as well as in low pressure lamps have a remarkably longer servicelife when made from substantially pure quartz glass than when made asheretofore from quartz glass selected solely for its ability to transmitthe rays involved.

It is, therefore, an essential object of this invention to providequartz glass casings for gas discharge tubes, said casings being madefrom quartz glass of a high degree of purity.

Thereby not only coarse particles of impurities that are visible to thenaked eye or under a low-power microscope must substantially be absentbut also impurities which are of very small particle size or are presentin the quartz glass in molecular magnitude and therefore not visible.

According to the present invention quartz glass is used which issubstantilaly free not only of conductive elements, such as metals, forinstance, copper, iron, alkali metals, or alkaline earth metals, orcarbon, but which is also substantially free of metalloids such asarsenic and tellurium and of compounds of such elements that exert anunfavorable influence upon the service life of quartz lamp casings madetherefrom. Although best results are achieved when using chemically purequartz glass, in practical operation it is not always possible toproduce such chemically pure quartz glass. The amount of impuritiespresent therein, however, must be the lowest possible amount achieved inordinary manufacture, i.e., the quartz glass must be substantially pure.

It is one of the characteristic features of the present invention touse, in the manufacture of quartz glass casings for gas discharge tubesand the like, a quartz glass the degree of contamination of which is sosmall that formation of seed-like formations or nuclei which will causesubsequent crystallization within the quartz glass is substantiallyeliminated and, due thereto, the devitrification temperature isconsiderably increased or, respectively, devitrification of casings madefrom such substantially pure quartz glass due to exposure to highservice temperature is considerably retarded. Surprisingly it was foundthat, with such a substantially pure quartz glass, the tendency of gasdischarge tubes to become discolored in operation. is also considerablyreduced.

In order to produce quartz glass or quartz glass casings of the requiredpurity, it is necessary to proceed in such a manner that any substantialintroduction of impurities into the quartz glass during the manufactureof said casings is excluded. To achieve said purpose, it is advisable touse the purest possible starting material, for instance,

pieces of pure rock crystal. Said starting material is then purified asfar as possible and the melting process and preferably the building upand drawing processes are carried out in such a manner that not only anysubstantial introduction of impurities into the quartz glass isexcluded, but also under conditions, for instance, in an atmospherewhereby foreign elements that may be present in the quartz glass arerendered harmless or eliminated and carried away.

Processes of manufacture are preferred which take place at comparativelyhigh temperatures, for instance, between 2000 C. and 2,400 C. At suchtemperatures impurities are separated or evaporated. Care must be taken,of course, that such high temperatures exert their purifying effectuniformly over the entire cross section of the molten quartz.

An especially advantageous process consists in melting and building upsuch substantially pure quartz glass in a gas atmosphere, for instance,in the atmosphere of a noble gas, such as argon or helium, or in theatmosphere of a gas like oxygen or chlorine or other halogens. Thepresence of such gases not only impedes introduction of foreign elementsbut gases, such as oxygen or halogens, especially'chlorine, react withthe contaminating elements and cause evaporation thereof in the form ofvolatile compounds.

' Thereby conditions and especially temperatures are chosen whereby thereactivity of said gases with the atoms composing the quartz glass to bepurified is insignificant.

Occluded gases as well as any compounds formed therewith are readilydriven oif and expelled by heating to a suitable temperature, ifnecessary, in a vacuum.

By this means it is possible to reduce the amount of impurities, evenwhen present in molecular or atomar size, to a minimum or tosubstantially eliminate the same.

'Should, in spite of proper precautions, larger particles of impuritiesfrom the furnace walls or drawing tools, for instance, impurities of asize visible to the naked eye, penetrate into the quartz glass, it ispossible to subsequently remove said coarse particles of impurities, forinstance, by locally heating the corresponding part of the shaped quartzcasing. Another way to render said coarse particles harmless consists inuniformly distributing the same throughout the quartz glass by diffusionor even, although not as advantageously, by mechanical homogenization.Such a procedure is also capable of reducing the formation ofvitrification seeds or nuclei which cause premature aging of the quartzlamp. Such homogenization process will 'be described in greater detailhereinafter.

The treatment with halogens and especially with chlorine is ofparticular advantage and causes a noteworthy advance in the art, and isdescribed more in detail in copending application Serial No. 767,363.

In principle, this process of chlorine treatment consists incontinuously passing finely comminuted rock crystal material or the likethrough a fusion zone, continuously directing a stream of chlorine inthe path of such continuously flowing rock crystal material to envelopsaid rock crystal particles in such chlorine gas, collecting the moltenparticles in a mass of substantially pure quartz glass, continuouslybuilding up such quartz glass mass from such molten'particles, andremoving and/ or further working up the resulting substantially purequartz glass to the desired articles, especially to casings for gasdischarge tubes.

- When melting and drawing quartz glass in order to produce casings ofgas discharge tubes, it is, above all, necessary to eliminate any freehydrogen that has not combined with the oxygen supplied to the reactionchamber or with any atmospheric oxygen present therein.

The presence of other foreign gases, of course, is a burden or dead loadon the process and impairs its thermal economy. It is, however, notnecessary to exclude foreign indiiferent gases, for instance, nitrogenor noble gases. Their presence, under certain circumstances, has theadvantage that embedding of other impurities in the quartz glass isrendered difficult or even completely prevented. The attached drawingsillustrate a preferred embodiment of apparatus for carrying out theprocess according to the present invention and of ultra-violet andilluminating lamps provided with casings made according to the presentinvention. In these drawings:

FIG. 1 shows a lamp for producing ultra-violet rays which serves for thegeneration of ultra-violet rays, said lamp having a casing made ofsubstantially pure quartz glass according to the present invention.

FIG. 2 illustrates a mercury vapor mixed light lamp, i.e., a lamp forlighting or illuminating purposes in which, besides the gas dischargeradiator, a coiled filament is arranged, said lamp also being providedwith a casing made of substantially pure quartz glass.

FIG. 3 illustrates a portion of a furnace for making a substantiallypure cylinder-like quartz glass body wherein a preferred formhomogenizing means is shown.

Chlorine treatment of quartz raw material in the molten state accordingto the description of copending application Serial No. 767,363considerably improves the properties of the resulting quartz glass andof casings of gas discharge tubes and lamps made therefrom with respectto their permeability to ultra-violet rays and their service life.

When proceeding according to that invention, foreign elements cannotbecome embedded in the quartz glass at the place where it is molten andbuilt up to a quartz glass body because said foreign elements formreadily volatile compounds with chlorine and escape in the form of suchcompounds. Thus, for instance, sodium, magnesium, calcium, barium,aluminium, copper, zinc, titanium, and/or iron ore are removed in theform of their chlorides. Due thereto, quartz glass casings according tothat invention exhibit a high resistance against recrystallizationbecause substantially no crystallization nuclei or seeds are present inthe quartz glass treated in this manner. This has the further advantagethat aging of tubes and lamps made with such casings is verysubstantially retarded. Devitrification and discoloration are alsoconsiderably retarded and the service life of gas discharge tubes andlamps provided with such casings is considerably prolonged.

The process according to that invention has the advantage that, whenworking with the indicated chlorine concentration, occlusion of freehydrogen or intercalation of hydroxyl groups into the structure ortexture of the quartz glass is avoided and said quartz glass is composedto a much greater extent of silicon dioxide or silicon tetroxidetetrahedrons than this is the case in any other known process of meltingrock crystals. The presence of such readily volatile gases as hydrogenin quartz glass as made heretofore has a reducing effect and causesformation of lower silicon oxides and especially silicon monoxides,which lower silicon oxides considerably impair the chemical propertiesand the transmissivity for rays of quartz glass containing such gases.

It is evident that the quality of quartz glass material of which casingsfor glass discharge tubes are made is not only dependent upon theimpurities which can be determined as residue, for instance, onevaporating quartz glass by means of hydrogen fluoride, but also uponits content of volatile substances and especially of hydrogen and ofcompounds which contain the same elements as silicon dioxide itself,i.e., lower oxides of silicon, such as silicon monoxide.

By subjecting quartz raw material to an especially intense chlorinetreatment as described in copending application Serial No. 767,363,quartz glass can be obtained which is free of any fluorescence.Fluorescence,

' as is known, is caused by the presence of centers of dis turbances inthe lattice structure of a substance. For instance, very small amountsof a foreign element present in a material are capable of causingfluorescence. Absence of fluorescence in quartz glass, therefore, can beconsidered as proof of highest purity.

Casings of gas discharge tubes composed of high purity quartz glassaccording to that invention ordinarily exhibit, due to their highpurity, a greater transmissivity to ultra-violet rays than casings madeof commercial quartz glass. Said transmissivity can be particularlyobserved in the-shortwave part of the spectrum up to about 200-300angstroms. An immediate result thereof is a higher yield of ultra-violetUVC (ultra-violet rays in the C range, i.e. up to about 2800 angstroms).

The absence of hydrogen in casings of gas discharge tubes and lampsproduces further advantages. Evacuation and heating time onmanufacturing tubes and lamps with quartz glass according to thisinvention is considerably reduced. Furthermore, in the operation of gasdischarge tubes and lamps made with unheated quartz glass, occludedhydrogen emanates from the casing walls and causes ionization of the gasdischarge path. These disturbances are completely eliminated when usingas casing material quartz glass according to the present invention whichis substantially free of hydrogen.

While it is advisable, when carrying out the present invention, toprevent any occlusion of hydrogen and/or embedding of lower siliconoxides, especially of silicon monoxide, in contrast thereto, inclusionof silicon chlorides into the silicon dioxide structure in the course ofthe chlorine treatment in accordance with the invention does not causedisadvantages. On the contrary, such inclusion is often of advantage.Especially suitable are the high molecular silicon chloride compoundsformed which are stable even at temperatures up to SOD-900 C. Such highmolecular silicon chlorides, incorporated into the structure of quartzglass, decrease its reactivity. Such decrease in reactivity causesconsiderable retardation of the onset of devitrification at highertemperatures. Consequently, the decrease in ultra-violet intensity, forinstance, in a mercury high-pressure lamp, sets in only after such lampsor tubes have been used for a considerably longer period of time thanlamps or tubes the casings of which are made of untreated quartz glassas used heretofore.

The use of chlorination-produced substantially pure quartz glassconsiderably improves the service life of ultra-violet tubes and lampsof various kinds, and especially of tubes and lamps for technical andilluminating purposes. For such purposes there have mostly been usedhigh pressure and highest pressure mercury discharge tubes andespecially lamps provided with an additional gas filling to causeignition. Usually noble gases are used as such filling. Such lamps havethe form of a tube of small cross-section or spherical or capillaryform. Such luminous-discharge lamps or tubes made of quartz glass areexposed at their inner walls to especially high thermal stress due totheir high energy density. The voltage drop per cm., for instance, is ofthe magnitude of several hundred volts, at a current intensity of themagnitude of several or only a few amperes. Thereby, a surface luminousintensity of the tied up gas discharge is produced which is of themagnitude of tens of thousands of international candles per sq. cm. Thetemperature in such tubes rises to 700 C. to 1200" C. (1292 F. to 2192F.).

With mercury-discharge tubes, as they were used heretofore in themanufacture of technical and therapeutic ultra-violet lamps, aging dueto etching or corrosion and discoloration of the quartz glass casingoccurs during prolonged use of such lamps. illuminating lamps asheretofore employed, however, exhibit devitrification and discolorationalready after a few hours of use. Such rapid onset of devitrificationand discoloration is the reason why heretofore the use of suchilluminating lamps was considered impractical.

The difierence in the service life of gas-discharge tubes as heretoforeemployed and gas-discharge tubes with quartz glas casings made accordingto the present invention is so remarkable that technical and therapeuticultraviolet lamps provided with burners according to this invention, andeven more so illuminating lamps provided with the new gas dischargetubes, open entirely new technical possibilities. It is thus possible,to considerably prolong the time when devitrification, seriouslyaffecting the output in light energy '(at least by 40%) sets in.Furthermore, progressive devitrificat-ion, after it has set in, isslowed down to such an extent that at the end of the normal service lifeof a high-pressure lamp the entire ultra-violet emission may decreaseonly as little as to while lamps as heretofore used may lose about 40%to 50% of said ultra-violet emissivity within the same A. range.

The new burners with gas discharge tubes have the additional advantagethat they turn out more uniformly during manufacture than heretoforepossible. Consequently, waste on large scale manufacture is reduced.Likewise, the working conditions during heating and evacuating the tubesare simplified.

Casings for gas discharge tubes obtained by the chlorine treatment,during building up of the quartz glass blank and/or during shaping,molding, drawing, or otherwise forming said casings, as described incopending application Serial No. 767,363, have the further advantagethat less shrinkage is observed on making such casings. This is a veryimportant feature of the present invention because it eliminates, to agreat extent, waste and rejects and, thus, allows more economicproduction of said casmgs.

Gas discharge tubes having quartz casings and envelopes are employed forthe generation of ultra-violet rays and of visible light. To utilizesuch tubes for generating ultra-violet rays has attained greatimportance for therapeutic as well as for technical purposes. Mercurydischarge tubes have proved to be especially suitable. Other metal-vapordischarge tubes, for instance, of cadmium, are of minor importance. Inorder to attain highest light-emission, it has been suggested to fillthe discharge tube with thighly volatile compounds, for instance, withchlorides of metals which are only difiicultly volatile. Said chloridesdissocaite in the discharge tube so that the characteristic spectrum ofthe metals is emitted.

in the case of mercury discharge lamps a distinction is made between lowpressure discharge lamps and high pressure discharge lamps, the borderline between both types being at a pressure of about 10 to torrs.Discharge lamps at pressures of more than about 30 atmospheres aredesignated as highest pressure discharge lamps. The present inventionrelates more particularly to the improvement of gas discharge tubes forhigh pressure discharge and, at the same time, for highest pressuredischarge.

Quartz glass casings for gas discharge tubes made according to theprocess of treating the starting material wth chlorine, as described indetail in application Serial No. 767,363, represent an approximatestandard for determining which amount of impurities is permissible insuch casings in order to guarantee an extraordinarily high service lifeof said casings and consequently of such gas discharge tubes.

Substantially higher amounts of impurities than those obtained by saidchlorine treatment are not permissible, although a small increase insuch impurities will not considerably shorten the eifective service lifeof such tubes. Regard-less, in what manner the quartz glass casings wereproduced and from what kind of quartz glass material they were made, itis readily possible to determine whether the quartz glass issufliciently pure by comparing said casings lwith casings made fromquartz glass treated with chlorine. When using for such other processesa starting material of the highest possible purity and observing theabove mentioned precautionary measures, it is possible to produce quartzglas casings of extraordinarily high efiective service life, comp-arablein their eifectiveness with quartz glass casings made from chlorinetreated quartz glass.

To increase the service life of casings for gas discharge tubes madefrom quartz glass which does not fully correspond to the above mentionedrequirements with regard to purity, i.e., the purity of which does notfully correspond to that of chlorine treated glass, said casings may besubjected to a homogenizing treatment. Said treatment which forms afurther object of the present invention consists in homogenizing quartzglass in such a manner that foreign elements included in such quartzglass are disributed as finely and uniformly as possible throughout thequartz glass mass. It is possible, in this manner, for instance, torender quartz glass produced by other methods than by chlorine treatmentapproximately equivalent in its quality and prolonged service life, whenused as casing for gas discharge tubes, to quartz glass subjected tochlorine treatment. It is also possible to still further improve thestability of guartz glass subjected to the chlorine treatment, describedabove more in detail, by homogenizing and uniformly and evenlydistributing therethrough any traces of impurities which might bepresent in such chlorine treated quartz glass.

Homogenizing is effected, for instance, by mechanically stirring themolten quartz mass. For this purpose the molten mass 38 to behomogenized is thoroughly worked, as shown in FIG. 3 of the attacheddrawings, in chamber 57 heated to melting temperature, by means ofstirring rods 59 and 6b which are moved relatively toward and away fromeach other. Progress of homogenization may be observed, for instance, byoptical processes. An especially advantageous process of homogenizingquartz glass is the dispersing process whereby quartz glass is exposedto the action or" supersonic waves.

The stirring process is preferably carried out in such a manner that anydirect contact of the material with the walls of the furnace, etc., isavoided, for instance, a body of quartz glass 58 is held at both ends bysuitable supports 59 and 60 and is rotated. Said supports 59 and 60 aremoved relatively toward and away from each other but in a manner thatany contact of the softened mass with the walls, for instance, ofheating chamber 57 is avoided. The body of quartz glass 53, freelysuspended and attached to rods 59 and 60, is introduced into cylindrical furnace 57 heated by an oxygen-hydrogen blow pipe or electricallyby resistance elements. The temperature of said furnace is so high thatsoftening of the quartz mass takes place. Said softened mass is thenalternately compressed and stretched by moving said rods relativelytoward and away from each other. A preferred mode of carrying out thisstirring process consists in twisting the rods against each other. Sincea furnace as used for such treatment usually has only a comparativelynarrow zone of highest temperature, the quartz glass body is advancedtherein step by step so as to treat its entire length. In this manner amass of completely homogenized quartz glass is obtained.

it is also possible to homogenize pieces of quartz glass, at least to alimited depth, by subjecting the same to a treatment with supersonicwaves. For this purpose the work piece 58 to be treated, as shown inFIG. 3 of the attached drawings, is attached by fusing to a quartz waresupport 53 at one end and to a piece of quartz glass 60 which isconnected to a magnetostriction oscillator (not shown) at the other end.

The short quart glass cylinder 58 to be subjected to the action ofsupersonic waves is placed into electric resistance furnace or highfrequency furnace 57 which is heated to a temperature of at least 1600'C. and preferably to a temperature between 1300" C. and 2200 C. Amagnetostriction oscillator (not shown) as well as an iron rod (notshown) serving as coupling element are cooled. The end of said iron rodhas a cup-like shape. A piece of quartz glass 66 is fit, by grinding,into said cup-like end and is clamped therein in such a manner that asatisfactory seal and contact are assured.

The quartz glass cylinder 58 held at its other end by quartz waresupport 59 is then exposed at the above indicated temperature to theaction of supersonic waves with a frequency of 10,000 hertz units to 30,000 hertz units, and preferably with a frequency of 15,000 hertz unitsand 20,000 hertz units. Since the depth of penetration to which thesupersonic waves penetrate the quartz glass cylinder is limited, thehomogenizing effects achieved thereby extend only to a certain depth ofthe heated quartz glass cylinder is limited, the homogenizing eifectsachieved thereby extend only to a certain depth of the 8 heated quartzglass cylinder. Thereby the particles of the quartz glass materialperform oscillating movements. Due to the pressure of the sound rays,limited local displacements or dislocations occur within the materialsubjected to said supersonic pressure.

As has been mentioned above, said homogenizing treatment causes uniformdistribution of the impurities in a quartz glass mass. Impuritiespresent in non-homogenized quartz glass readily senve as seed-likeformations or nuclei which are responsible for more or less pronouncedrecrystallization and, consequently, for devitrification and,furthermore, for certain reduction phenomena causing discoloration. Whenheating pieces of such non-homogenized quartz glass, for instance, to1000 C., small areas or spots become visible after about 10 hoursexposure to such a temperature, whereas, before such heating, such spotscould not be detected by the naked eye or, if they could be detected atall, then only by a sensitive striae method. Said spots increase in sizeon continued heating. After heating for several weeks they form largerecrystallization centers having a diameter up to many millimeters. Ofcourse, speed and extent of devitrification is dependent also on otherfactors.

More exactly speaking, two ditferent groups of inclusions of foreignelements must be distinguished, namely such which are not at all or onlyslightly affinitive to silicic acid. Such impurities having a particlesize above and up to 1000 such as particles of carbides or particles ofdiflicultly soluble elements, are reduced in size to a particle sizebelow l00,u and preferably below 10,11. by the above describedhomogenizing treatment. As a result thereof the tendency of saidparticles to act as seeds or nuclei causing harmful reactions isconsiderably reduced.

The other type of impurities are those which are capable of formingcompounds with quartz glass and of interlinking therewith to a glasscontaining larger or smaller amounts of silicic acid. Such impuritiesare certain metal oxides, especially oxides of alkaline earth metals.Thereby frequently mixed glasses with a particle size up to 3 mm. andeven more are formed. Said mixed glasses are not directly visible to thenaked eye but can be detected only by interference, striae, orpolarization methods. Such embedded particles of mixed glass, of course,rapidly crystallize on heat treatment and form thereby devitrificationcenters. The reactivity of homogenized quartz glass is the lower themore such inclusions are interlinked and the more thoroughly theinterlinked glass complex is distributed within the silicic acidlattice. Uni form distribution of the various reduction products, suchas silicon monoxide and other stages of reduction, is also of importancefor the production of quartz glass of superior quality and is alsoachieved by homogenization as described above.

It shall, of course, be left open to question whether besides the aboveindicated effects, other effects are additionally produced by ahomogenizing treatment according to this invention and whether saidother effects are also responsible for the improvements achieved withrespect to aging, and especially to devitrification and reductionphenomena causing discoloration.

The discharge chambers, the so-called burners, in high pressure lampsand highest pressure lamps ordinarily consist of quartz glass. In mostcases the casing of the burner is a tubular body, usually of elongatedshape or bent in U-form. Of course, devices of this type of sphericalshape or provided with spherical extensions are also known. Capillarytubes are also used especially when high energy concentration of the gasdischarge is required. The casings or envelopes of such burners are madeof quartz glass tubes, which were drawn, for instance, from the blankthat was fused by a gas or electric process.

An example of a burner useful for the production of ultra-violet rays,especially in therapeutic lamps,

which, however, may be used for many other technical purpose isillustrated in FIG. 1. It represents a mercury high pressure lamp, thequantity of mercury present therein being regulated in such a way thatit completely vaporizes during operation. Electrodes 43, supplied withelectric current by means of current inlets 44, are arranged in quartzglass casing 45, according to this invention. Screen 46 preventsimpurities from passing from the electrodes into the gas chamber. Casing45 consists of quartz glass which has been molten and drawn according tothe above described process of this invention in the presence ofchlorine gas, and thus, is substantially pure. As already mentioned, theimprove ment achieved by this invention is of considerable importancefor ultra-violet radiators not only for therapeutic use but also forvarious technical purposes, for instance, in connection with variousphysical apparatus, laboratory devices, and as source for ultra-violetrays for chemical reactions. The improvement according to this inventionis especially useful in connection with high pressure lamps in devicesfor making blueprints because such devices require as uniform emissionas possible over the entire length of the burner.

In FIG. 2 a lamp for illuminating purposes is illustrated which containsa gas discharge lamp according to the present invention. Said lamp is amercury vapormixed light lamp, i.e., a lamp combining a mercury vapordischarge tube with a coiled up filament lamp, both serving at the sametime as a source of light. Burner casing 47 is made of quartz glass. Twoelectrodes 48 and auxiliary electrode 49 provided to insure perfectignition, and ignition resistance 50 are arranged in said lamp. Coiledup filament 51 fulfills a double task. It serves as a further source oflight and, at the same time, as a series resistance for the gasdischarge tube. Parts supplying the electric current are conductor 52,which also serves as a support, pinch foot 53 for vacuum tightintroduction of the electric wire, and screw cap 54 for screwing in thebulb. All the parts mentioned are sealed in glass bulb 55. In electriclamps for illuminating purposes frequently the gas discharge tube is notsurrounded by a coiled up filament and the series resistance is arrangedoutside of the lamp body. In this case, for instance, the seriesresistance may be a simple ohmic resistance, or an inductive resistor,or a filament lamp. All these illuminating lamps according to thepresent invention have a guaranteed considerably increased service life.

The above described devices are, of course, merely examples ofembodiments of this invention and serve to demonstrate the advance inthe art achieved thereby. The invention, of course, is by no meanslimited thereto and may find various other applications.

Instead of building up a substantially pure quartz glass suitable forproducing casings for gas discharge tubes by means of silicon dioxide,it is also possible to use other silicon compounds which decompose athigh temperature to form silicon dioxide, and particularly siliconcompounds which are hydrolytically decomposed to silicon dioxide.Silanes which react with steam or oxygen to form silicon dioxide are,for instance, suitable for certain purposes. The conditions regardingthe treatment with chlorine or chlorine compounds correspond to thosedescribed above for directly building up quartz glass from silicondioxide. Use of organo-silicon compounds as starting materials is quitesatisfactory for many purposes.

It is advisable to use very pure silicon dioxide as starting materialfor carrying out the invention in the manner described above and,furthermore, to take care that during the various steps of processingimpurities are prevented from getting into the quartz glass. Proceedingin this manner will insure that casings for gas discharge tubes of thehighest purity exhibiting all the above mentioned advantages areobtained. The process according to the present invention, however, hasthe further ad- 10 vantage that it is also possible to use somewhat lesspure starting materials, such as rock crystals having included therein,for instance, certain metal oxides and/ or silicates. Such impuritiesare also substantially completely removed by subjecting said impurestarting material to the process of this invention.

Substantially pure quartz glass, obtained according to the abovedescribed treatment process, can be advantageously used not only formaking casings for gas discharge tubes or lamps but also for otherapparatus and parts of apparatus requiring quartz glass of a qualitysatisfactory to optical requirements. Such parts of apparatus comprise,for instance, transparent media which bundle rays in order toconcentrate or beam them or to disperse them or which serve forrectilinear propagation of parallel directed radiation. Such transparentmedia comprise, for instance, prisms which deflect a rectilinear path ofrays in such a manner that the light is spectroscopically dispersed orseparated, or lenses which collect light concentrically around anoptical axis or which disperse light whereby refracting influences maycompensate each other. Besides prisms and lenses, such parts of opticalapparatus and devices comprise also plane plates which are used, forinstance, for making filters and filter cuvettes or for sealing chambersfrom which a path of optical rays is to be conducted into an areasubjected to other conditions of pressure.

Various types of glass and crystals were used heretofore for suchtransparent media. Thereby it is of great importance that opticaldisturbances, such as striae, occlusions or inclusions which change therefractory power of said materials or which absorb optical rays, aresubstantially avoided.

This invention produces quartz glass of considerably higher purity thanquartz glass as heretofore used for parts of optical and the likeapparatus and devices. Said higher purity is also responsible for thehigher transparency to ultra-violet light of parts of optical apparatusmade therewith.

The transparency of quartz glass reaches into the zone of 1600angstroms. In the range between about 2200 angstroms and 1600 angstroms,transparency decreases quite abruptly. For instance, the transparency ofa normal quartz glass plate of 10 mm. thickness at 2200 angstroms isabout 35% and at 1600 angstroms about 10%. When using glass plates ofthe same thickness made from quartz glass, treated for instance withchlorine according to the present invention, the transparency toultra-violet rays at 2000 angstroms is about 85%, at 1800 angstromsstill about and at 1700 angstroms about 30% to 50%, i.e., at least aboutthree times as high as that of ordinary quartz glass heretofore used inoptical apparatus and parts of optical apparatus and the like. Due tothe higher purity of quartz glass treated with chlorine or according tothe present invention, such optical apparatus and parts of opticalapparatus and the like are usually substantially free of striae and ofinhomogeneities.

Chlorine treatment may be additionally carried out during subsequenthomogenizing of and working and processing said quartz body or mass todesired parts of optical apparatus. Working up, further processing,shaping, molding, or otherwise forming articles, parts or products fromthe quartz glass treated according to this invention is carried out bythe standard methods. In such standard methods, for instance, the knownoperations of pressmolding, upsetting, and stretching of quartz glasscan readily be performed in the presence of chlorine. The chlorinetreatment can also be combined with the homogenization of the moltenquartz glass as heretofore described, i.e., whereby striae and opticalinhomogeneities which are present in said quartz glass, are eliminated.Chlorine treatment during homogenization has the advantage that anextraordinarily intimate contact even of the smallest quartz particleswith chlorine is insured.

We claim:

1. In the process for making quartz glass casings for ultraviolet lampsfrom quartz glass containing amounts of impurities, selected from thegroup consisting of the conductive elements, carbon, metalloids,compounds containing such elements, including the oxides of silicon witha lower oxygen content than silicon dioxide, and hydrogen normallycapable of forming centers of recrystallization including the steps, inwhich the glass is heated to a melting temperature, shaped in the formof a casing and allowed to solidify in this form, the improvement whichessentially consists of the combination with said steps of reducing theparticle size of said impurities to below about 100,u., and homogenizingand uniformly dispersing the same throughout said quartz glass prior tosolidification in the form of said casing.

2. Improvement, according to claim 1, in which the particle size of saidimpurities is reduced to below about 10p" 3. Improvement, according toclaim 1, in which said homogenization and dispersion of said impuritiesis effected by placing the melted quartz glass mass between supports ina zone heated to the melting temperature of 1.2 said quartz glass massand moving said supports while the mass is supported thereby relativelytoward and away from each other.

4. Improvement, according to claim 1, in which said homogenization anddispersion is effected bysubjecting the molten quartz glass mass to theaction of supersonic waves.

References Cited in the file of this patent UNITED STATES PATENTS1,003,271 Kent Sept. 12, 1911 1,549,597 Miller Aug. 11, 1925 1,612,628George Dec. 28, 1926 2,089,546 Dudding et al Aug. 10, 1937 2,236,708Grimditc-h -1 Apr. 1, 1941 2,353,668 Hinman July 18, 1944 2,367,595Marden Jan. 16, 1945 2,568,459 Noel Sept. 18, 1951 2,794,301 Law et a1June 4, 1957 2,904,713 Heraeus et al Sept. 15, 1957 FOREIGN PATENTS254,502 Switzerland Dec. 16, 1948

1. IN THE PROCESS FOR MAKING QUARTZ GLASS CASINGS FOR ULTRAVIOLET LAMPSFROM QUARTZ GLASS CONTAINING AMOUNTS OF IMPURITIES, SELECTED FROM THEGROUP CONSISTING OF THE CONDUCTIVE ELEMENTS, CARBON, METALLOIDS,COMPOUNDS CONTAINING SUCH ELEMENTS, INCLUDING THE OXIDES OF SILICON WITHA LOWER OXYGEN CONTENT THAN SILICON DIOXIDE, AND HYDROGEN NORMALLYCAPABLE OF FORMING CENTERS OF RECRYSTALLIZATION INCLUDING THE STEPS, INWHICH THE GLASS IS HEATED TO A MELTING TEMPERATURE, SHAPED IN THE FORMOF A CASING AND ALLOWED TO SOLIDIFY IN THIS FORM, THE IMPROVEMENT WHICHESSENTIALLY CONSISTS OF THE COMBINATION WITH SAID STEPS OF REDUCING THEPARTICLE SIZE OF SAID IMPURITIES TO BELOW ABOUT 100U, AND HOMOGENIZINGAND UNIFORMLY DISPERSING THE SAME THROUGHOUT SAID QUARTZ GLASS PRIOR TOSOLIDIFICATION IN THE FORM OF SAID CASING.